The mechanisms that control severe and progressive pulmonary fibrosis remain poorly understood. This program has focused on elucidating the roles of the extracellular matrix in regulating lung inflammation and fibrosis in the context of non-infectious lung injury. Work from our laboratory during this period of funding has focused on the role of the extracellular glycosaminoglycan hyaluronan (HA) in regulating lung inflammation and fibrosis. We have identified distinct functions for HA depending on both the cellular context of its expression and the form in which it is presented to interacting cells. We have discovered that HA expressed on the cell surface of lung epithelial cells serves a protective function against non-infectious insults. In contrast, when myofibroblasts are directed to over-express hyaluronan synthase 2 (has2) under control of the alpha-smooth muscle actin promoter (ASMA-HAS2), the result is a severe and progressive fibrodestructive lung disease after injury that causes significant mortality. We have made the exciting observation that fibroblasts isolated from ASMA-HAS2 mice have an invasive phenotype. We have also generated the first mesenchymal-targeted deletion of has2 expression and found that both fibrosis and matrix invasion of fibroblasts is blunted. Based on these data we have generated the hypothesis that severe and progressive lung fibrosis requires the development of an invasive fibroblast phenotype dependent upon has2. We will test this hypothesis in the following Specific Aims: 1. Determine the roles of has2 in regulating progressive lung fibrosis and the development of an invasive fibroblast phenotype. 2. Determine the role of has2 in regulating the development of pulmonary fibrosis in TGF- transgenic mice. 3. Determine the source of the invasive fibroblast phenotype using lineage labeling of alveolar epithelium, mesothelium and resident lung fibroblasts. 4. Identify genomic signatures of matrix-invading fibroblasts from severe pulmonary fibrosis and elucidate the signaling pathways that regulate invasion.